The present disclosure relates generally to semiconductor devices and, more particularly, to backside-illuminated image sensors. U.S. application Ser. No. 11/424,286 filed Jun. 15, 2006, and U.S. application Ser. No. 11/753,480 filed May 24, 2007 and granted as U.S. Pat. No. 7,656,000, are hereby incorporated by reference and may provide additional details regarding image sensors relevant to the present disclosure.
In semiconductor technologies, backside-illuminated sensors are used for sensing a volume of radiation (e.g. light) projected towards the back surface of a substrate. To do this, an image sensor device uses an array of image sensor elements (e.g. pixels). Each image sensor element includes at least one radiation sensing element, described herein, as a photodetector. The photodetectors may be formed on the front side of the substrate, the substrate being thin enough to allow the radiation incident on the back surface of the substrate to reach the photodetectors. A thin substrate may be advantageous in reducing optical and/or electrical cross-talk between image sensor elements. However, a thin substrate may also degrade the sensor performance, for example, by degrading quantum efficiency (e.g. its ability to convert radiation into a signal) in particular for long wavelengths of visible light (e.g. 650 nm). Each image sensor element also typically includes at least one transistor coupled to the photodetector. Such transistors must be optimized for example to prevent “punch through.”
As such, an improved backside-illuminated image sensor is desired.